This invention relates generally to television receivers and particularly to television receivers incorporating IF filters that utilize surface acoustic wave (SAW) devices. SAW filter devices, or transducers, are well known in the art and are acknowledged to be very stable and predictable in performance. The SAW transducers comprise comb elements formed on lithium niobate substrates, the surface wave propagation of which, along with the comb configuration, determines the filter response. A SAW resonator comprises an input comb and one or more resonant reflector combs. It is also known in the art to use a SAW resonator as the "tank circuit" of an oscillator. The temperature coefficient of one type of lithium niobate substrate material is about 92 PPM per degree centigrade. While this is not excessive, it can cause a noticeable shift in tuning of a resonator or filter. Another problem exists in that the SAW filter and resonator devices may exhibit manufacturing tolerance variations. The combined effects of tolerance variations and shifts in characteristics with temperature changes often result in the center frequency of a SAW filter changing. To maximize performance, any change in the center frequency of a SAW filter should ideally be compensated by a similar change in the frequency of the IF picture carrier developed.
The invention permits the IF picture carrier to automatically track the SAW filter center frequency.